1. Field of the Invention
The present invention relates to a system for calculating the remaining charge of a battery used in an electronic device such as a portable electronic device.
2. Description of the Related Art
Portable electronic devices use batteries to supply power for the operation of the device. As such portable electronic devices there are, for example, digital cameras, video cameras, mobile phones and the like.
A portable electronic device like those described above is constructed so as to accurately grasp the actual remaining charge of a battery. For example, in JP-A-2004-150951, a structure in which serially connected resistors and switching elements are connected in parallel to the battery is disclosed. The structure comprises a battery control means that acquires information relating to changes in the voltage of the battery detected by a battery voltage detection means by switching the switching elements on and outputting an electric current from the battery to the resistors. The battery control means determines the remaining charge of the battery on the basis of the information relating to changes in the voltage of the battery (see JP-A-2004-150951).
A portable electronic device like that described above, when carrying out an operation that consumes a lot of power, only begins to do so after determining whether or not such an operation is feasible with the present remaining battery charge. One example of a conventional remaining battery charge detection circuit provided for this purpose is shown in the block diagram of FIG. 16, using the example of a digital camera.
A description will now be given of the operation of the conventional remaining battery charge detection circuit, using FIG. 16.
As shown in FIG. 16, a battery voltage detector 506 and a constant current load unit 505 capable of outputting a constant current are connected in parallel to a battery 501. The constant current load unit 505 is constructed so as to be susceptible to ON/OFF control. The battery voltage detector 506 comprises voltage-dividing resistors R1, R2 serially connected.
A power supply unit 502 that supplies power to a digital camera system 503 converts the voltage of the battery 501, generates multiple types of voltages required by the digital camera system 503, and supplies power. A controller 504 controls the digital camera system 503 as a whole. It should be noted that FIG. 16 shows only the blocks relating to the remaining battery charge detection circuitry.
A power supply control unit 510 controls the power supply unit 502, switching ON the voltage supplied from the power supply unit 502 deemed necessary to operate the digital camera and switching OFF the voltage deemed unnecessary. Operating switches 508 include a release switch, a mode switch for selecting an operating mode and the like, for inputting operating instructions.
The operating switches 508 are connected to a switch state detector 509 that monitors the state of the operating switches 508. A constant current load control unit 511 switches the constant current load unit 505 ON/OFF depending on information relating to the switch state as detected by the switch state detector 509.
The battery voltage detector 506 causes a divided battery voltage detection signal to be supplied to an A/D converter 512, where the battery voltage information is converted into a digital value by the A/D converter 512. A remaining charge calculator 514 then calculates the remaining battery charge using the battery voltage information supplied from the A/D converter 512 and coefficients provided for each operating state and recorded in a coefficient recording unit 513. A remaining charge display unit 507 displays the remaining battery charge results calculated by the remaining charge calculator 514.
Next, a description is given of the operation of a remaining battery charge estimation circuit constructed as described above, while referring to the flow chart shown in FIG. 17.
Once the main power to the remaining battery charge estimation circuit is thrown and the circuit begins to operate, initially, it is determined whether or not a camera operating instruction has been input using the operating switches 508 (step S601). If the results of the determination indicate that a camera operating instruction has not been input, the camera stands by until such input.
If the results of the determination made in step S601 indicate that a camera operating instruction has been input, then the digital camera system 503, through the battery voltage detector 506, acquires present battery voltage information V0 digitally converted by the A/D converter 512 (step S602).
After acquiring the present battery voltage information V0, the constant current load control unit 511 switches the constant current load unit 505 ON and a predetermined load current IL flows from the battery 501 (step S603). The digital camera system 503 then acquires battery voltage information V1, which is the battery voltage at this time, digitally converted by the A/D converter 512 (step S604). After acquiring the battery voltage information V1 in step S604, the constant current load control unit 511 switches the constant current load unit 505 OFF (step S605). The following calculation process and comparison are then carried out at the remaining charge calculator 514 (step S606):Vth<V0−KΔV  (1)ΔV−=V0−V1  (2)
where Vth is a predetermined lower limit threshold voltage at which the camera can operate and K is an arbitrary operating mode coefficient predetermined for each camera operation input using the operating switches 508, which is stored in the coefficient recording unit 513.
If as a result of the calculation process and comparison described above and carried out at the remaining charge calculator 514 (step S606) it is determined that the results of the calculation process are greater than the lower limit threshold voltage Vth, it is then determined that the operation input in step S601 can be executed, operation is started (step S607), and, when the desired operation is completed, operation is ended (step S608).
If as a result of the calculation process and comparison carried out in step S606 described above it is determined that the results of the calculation process are equal to or less than the lower limit threshold voltage Vth, it is then determined that the operation input in step S601 cannot be executed and a replace-battery sign is displayed on the remaining battery charge display unit 507 (step S609). Next, the power supply control unit 510 switches the power supply unit 502 OFF (step S610). Documents that describe this configuration include JP-A-2004-61393 and JP-A-2004-150951.
In general, the smaller the drop voltage ΔV the greater the relative proportion of A/D conversion quantification error and error due to non-uniformity in the components of the battery voltage detector 506. As a result, the smaller the drop voltage ΔV the lower the accuracy of the remaining battery charge calculation, and conversely, the greater the drop voltage ΔV the higher the accuracy of the remaining battery charge calculation. Moreover, since the drop voltage ΔV increases in proportion to the load power (PL=battery voltage×load current IL), the accuracy of the remaining battery charge calculation is proportional to the load power PL.
By contrast, it is known that, as shown in the characteristics graph showing the relation between voltage and running time in FIG. 18, with the battery 501 typically used in a portable electronic device such as a digital camera the battery voltage falls as the remaining charge declines. Therefore, connecting the constant current load unit 505 to the battery voltage as in the conventional arrangement reduces the load power PL when the remaining battery charge is low, which is just when high detection accuracy is needed most. As a result, remaining battery charge detection accuracy declines.
Conversely, even when the remaining battery charge is adequate and the battery voltage is high, a remaining charge estimate is always performed prior to operation using the predetermined load current IL. When the battery voltage is high the load power PL increases, which causes a large power loss due to the remaining battery charge calculation operation.